The heat-sensitive recording method, by which colored images can be recorded by closely contacting a thermal head on the surface of a recording material, which has a heat-sensitive coloring layer prepared on a supporting base, and transferring thermal energy directly to the heat-sensitive layer or through a protective layer, is widely known and applied to facsimile receivers, printers and other devices.
However, such heat-sensitive recording method has a disadvantage, in that images cannot accurately be obtained. Further, the thermal head is worn out as residue from the heat-sensitive recording material adheres to the thermal head surface, because the thermal head is driven on the recording material with its head closely contacting the recording material, or the thermal head is damaged.
Such a heat-sensitive recording method using a thermal head also has another disadvantage that when recording at high speed or with high density, quality is limited, since high speed control on heating or cooling of the heating element is limited due to its structural characteristic.
To eliminate such disadvantages of the heat-sensitive recording method using a thermal head as described above, it has been proposed to make thermal recordings on the heat-sensitive material without contacting the recording material, at high speed and high density using laser light (for example, JP-A-50-23617 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-54-121140, JP-A-57-11090, JP-A-58-56890, JP-A-58-94494, JP-A-58-134791, JP-A-58-145493, JP-A-59-89192, and JP-A-62-56195).
However, in such a recording method using laser light, there is a disadvantage in that the thermal energy required for color forming cannot be obtained unless laser output is considerably large, because it is difficult for the heat-sensitive color forming layer to absorb visible light and light near the infrared region. Therefore the manufacture of compact and inexpensive devices is extremely difficult.
Though JP-B-50-774 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes a method to record on the recording material by irradiating with powerful light a raw paper coated with microcapsules enclosing ink to eject the ink out of the capsules, this method has not yet been put into practical use due to its very low sensitivity.
Thus, there have been many proposals for enabling a heat-sensitive recording layer to efficiently absorb laser light. To effectively absorb laser light, a light absorbing substance matching the wavelength of the laser light can be added to the heat-sensitive recording layer. In this case, the light absorbing substance added should be white, otherwise only recordings of low contrast and less quality will be obtained, because the background of the recording material is colored.
Many of the white, light absorbing substances are inorganic compounds in general, but most of them are low in absorptivity. Therefore, development of a less colored organic compound having high absorptivity is desired.
However, organic compounds which absorb light in the visible light region have colors in general, and their absorptivity will be higher when the color is more dense. Therefore, addition of such a light absorbing substance to the heat-sensitive recording layer (hereinafter referred to as the heat-sensitive layer) enhances sensitivity of the heat-sensitive layer, but improving the whiteness of the recording materials is difficult.
There are colored organic compounds (dyes) which can absorb infrared light when in an aqueous solution since they have a maximum absorption wavelength in the visible light region, but their maximum absorption wavelength in the visible light region will shift to longer wavelengths or infrared by aggregation of the dyes when the solution is dehydrated and in the desired state.
Because the heat-sensitive layer, which is colored at the time of coating, becomes colorless after the layer has dried, when such dyes are added to the heat-sensitive layer, the heat-sensitive recording material can be nearly white or colorless, and in addition, the infrared laser light absorptivity of the recording layer can be improved.
The present inventors carried out recordings using infrared laser light on a heat-sensitive recording material having a heat-sensitive layer which includes a coloring component and a specific tricarbocyanine dye, and found that this method can produce satisfactory results, which led to the present invention.
Therefore, the object of the present invention is to provide a heat-sensitive recording material for laser light recording which does not easily become colored and which provides high quality recordings.